The invention relates to switches for electrical signals, and, more particularly, to switches and systems for high frequency signals with micromechanical switch elements.
Typical rf switches are p-i-n diodes, but p-i-n diodes have problems including power consumption (the diode must be forward biased to provide carriers for the low impedance state), high cost, and nonlinearity.
Peterson, Dynamic Micromechanics on Silicon: Techniques and Devices, 25 IEEE Tr.El.Dev. 1241 (1978), includes silicon micromechanical metal-coated cantilevers which act as metal-to-metal switches. FIGS. 1a-b illustrate in plan and cross sectional elevation views of such switches with silicon dioxide ("oxide") cantilever 102 extending out over 7 .mu.m-deep opening 104 etched in silicon substrate 106. Metal electrodes 108-109 extend onto cantilever 102, and metal conductor 110 extends onto and up and out over the end of cantilever 102. Metal contact 120 on oxide 112 lies in the same plane as canitlever 102 and extends out under the end of conductor 110. The switch operates as follows. With no voltage applied between electrodes 108-109 and silicon substrate 106, cantilever 102 remains parallel to the surface of silicon substrate 106 and the switch is open. However, about 60 volts applied between the electrodes and the substrate pulls cantilever 102 towards substrate 106 until the end of conductor 110 makes contact with metal 120. This closes the switch. Release of the pull down voltage then opens the switch.
Micromechanical spatial light modulators with metallized polymer membranes appear in Hornbeck, U.S. Pat. No. 4,441,791. FIG. 2a shows a cross sectional elevation view through two pixels of an array of such pixels, and FIG. 2b shows the equivalent circuit. Voltage applied between the metal film 30-31 on the underside of polymer membrane 35 and the underlying electrode 21 pulls the membrane part way down to the electrode and thereby disrupts the flat surface of reflecting metal film 26 on the polymer membrane and thereby modulates reflected light. Applying too large a voltage collapses the polymer membrane down to the electrode and destroys the pixel. Selectively applying voltages to pixels in the array permits spatial light modulation.
Spatial light modulators with pixels made of metal torsion beams with landing pads appears in Hornbeck, U.S. Pat. No. 5,061,049. FIGS. 3a-d show such a pixel in perspective, cross sectional and plan views with metal beam 30 suspended by thin metal torsion hinges 34 and 36 over underlying electrodes 42 and 46 plus landing pads 40-14 41. With no voltage applied between electrodes 42, 46 and beam 30, the beam remains parallel to the metal surface 26, 28 as in FIG. 3b. A voltage applied between electrode 42 and beam 30 pulls on the beam and the beam twists counterclockwise (in FIG. 3d) on hinges 34, 36 and one beam edge moves down toward the electrode while the other beam edge rises as illustrated. Beam 30 stops when it contacts landing pad 40, which is at the same voltage as the beam (typically, ground). With beam 30 in this tilted position, light reflects at a different angle from the beam than light reflecting from the surface 28. Removal of the applied voltage allows hinges 34, 36 to relax and return beam 30 to the position parallel to the surface 28. Thus an array of such pixels can act as a spatial light modulator.
Further, applying a voltage between electrode 46 and beam 30 (and no voltage between electrode 42 and beam 30) will analogously twist beam 30 in the clockwise direction until it makes contact with landing pad 41. This provides a second angle of reflection for incident light.